LECTURE 7 Chlor_Alkali_Industries

Report
LECTURE 6
CHLOR ALKALI INDUSTRIES –
SODA ASH, CAUSTIC SODA,
CHLORINE
Chapter 13 in Shreve’s Chemical Process Industies
CHLOR
ALKALI
INDUSTRIES
• Caustic soda, soda ash and chlorine
• Rank close to H2SO4 and NH3 in
magnitude of $ value of use
• Lot of consumption in making other
chemicals.
• Uses – Soaps, detergents, fibers and
plastics, glass, petrochemicals, pulp n
paper, fertilizer, explosives, solvents
and other chemicals
CAUSTIC
SODA – NAOH
• Previously made by Causticization of soda
ash with lime
Na2CO3 + Ca(OH)2 → 2 NaOH + CaCO3
• Only 10% NaOH solution obtained
• Electrolysis of Brine – Most popular
method adopted nowadays.
CAUSTIC
SODA – NaOH
• Brittle white solid
• Readily absorbs moisture and CO2 from
air
• Sold on basis of Na2O content
• 76% Na2O equivalent to 98% NaOH
• Uses – Soaps, textiles, chemicals,
petroleum refining, etc.
USES OF
CAUSTIC
SODA
MANUFACTURE
OF NAOH
• Electrolysis of Brine
• Chlorine at Anode; Hydrogen along with
alkali hydroxide at cathode
• Three types of cell exist:
– Mercury Cell
– Diaphragm Cell
– Membrane Cell
• Raw Materials
1. Brine (NaCl)
2. Electricity
ENERGY
CHANGES-GIBBS
EQUATION
• Energy consumed in electrolysis is
product of current flowing and
potential of cell
• Gibbs Helmholz equation represents
the relation between electric energy
and heat of reaction:
HEAT OF
REACTION
(∆H)
• Found from heats of formation of the components of
the overall reaction:
• This reaction is broken down into following reactions
for formation:
Net ∆H for the overall reaction results from
VOLTAGE
EFFICIENCY
• ∆H is computed in Gibbs Helmholz
equation to get E = 2.31 V
• Voltage Efficiency = Epractical÷ETheoretical×100
• Generally range from 60 – 75 %.
• Faraday’s Law: 96,500C of electricity
passing through a cell produce 1 gm.eq. of
chemical reactions at each electrode
• Actually higher – Side reactions
• Ratio of theoretical to actual current
consumed is current efficiency (≈ 9597%)
CURRENT
EFFICIENCY • Current divided by area on which
AND ENERGY
current
acts
is
current
density
–
high
EFFICIENCY
value desirable
• Product of voltage efficiency and
current efficiency is energy efficiency
of cell
DECOMPOSITION
EFFICIENCY
• Ratio of equivalents produced in the
cell to equivalents charged
• Usually about 60 – 65 %.
• Diaphragm cells have very high
decomposition efficiencies
• But encounter difficulties with migration of
hydroxyl ions back to anode  formation of
hypochlorite ion
• At anode, OH- ions give
• Oxygen formed reacts with graphite anode,
decreasing its life
• In Metal anodes, oxygen does not react.
CELL TYPE
• Previously mercury was most widely used
• Health and environmental problems with
mercury discharge in nearby waters
• Improved designs of membrane cells and
cheaper purification techniques have
reduced cost and improved efficiencies
– Dominate the field nowadays
DIAPHRAGM CELLS
• Contain a
diaphragm made of
asbestos fibers to
separate anode
from cathode
• Allows ions to pass
through by
migration
• Graphite anode and
cast iron cathode
ASBESTOS
DIAPHRAGM
DIAPHRAGM
CELLS
• Diaphragm Permits the construction of
compact cells of lowered resistance as
the electrodes can be placed close
together
• Diaphragms become clogged with use
and should be replaced regularly
• Diaphragm permits flow of brine from
anode to cathode and thus greatly
lessens side reactions
• Cells with metal cathodes rarely get
clogged diaphragms and operate for 1-2
years without requiring diaphragm
replacements.
DIAPHRAGM
CELLS–
ADVANTAGES &
DISADVANTAGES
• Major Advantage – Can run on dilute
(20%), fairly impure brine
• Dilute brine produces NaOH 11% (NaCl
15%)
• Consumes lot of energy for
evaporation
• For 1 ton of 50% caustic need 2600 kg
of water to be evaporated.
• Some amount of Chloride ion remains
and is highly objectionable to some
industries (Rayon)
MEMBRANE CELLS
• Use semipermeable
membrane to separate
anode and cathode
compartments.
• Separate compartments
by porous chemically
active plastic sheets; that
allows sodium ions to pass
but reject hydroxyl ions.
MEMBRANE
CELL
MEMBRANE
CELL
ADVANTAGES
OF
MEMBRANE
CELL
• Purpose of membrane is to exclude OH- and
Cl- ions from anode chamber
• Thus making the product far lower in
salt than that from a diaphragm cell
• Membrane cells operate using more
concentrated brine and produce purer, more
concentrated product
• (30-35% NaOH containing 50 ppm of
NaCl)
• Requires only 715 kg of water to be
evaporated to produce 1 M ton of 50% NaOH
ADVANTAGES
OF
MEMBRANE
CELL
• Because of difficulty and expense of
concentration and purification, only large
diaphragm cells are feasible
• Membrane cells produce conc NaOH
• considerable saving in energy
(Evaporation)
• and saving in freight (operate to the point
of caustic use)
• Small, efficient units may cause a revolution in the
distribution of the chlor-alkali industry,
particularly if efficiencies remain high
DISADVANTAGE
OF MEMBRANE
CELLS
• Membranes are more readily
clogged than diaphragms, so
some of savings are lost, bcos
of necessity to pretreat the
brine fed in order to remove
Ca and Mg before electrolysis
MERCURY
CELLS
• Operate differently than the other two
• Cathode is a flowing pool of mercury;
graphite anode
• Electrolysis produces a mercury-sodium
alloy (amalgam)
• Amalgams is decomposed in a separate
vessel as:
2Na.Hg + 2H2O → 2 NaOH + H2 + Hg
ADVANTAGES AND
DISADVANTAGES
OF MERCURY
• 50% NaOH is produced with very
low salt content (30 ppm)
• No evaporation needed
• Small loss of mercury to
environment poses severe
problems.
MERCURY
CELL
MERCURY
CELL
UNIT
OPERATIONS
AND
CHEMICAL
CONVERSIONS
•
•
•
•
•
•
Brine Purification
Brine Electrolysis
Evaporation and Salt Separation
Final Evaporation
Finishing of Caustic
Special Purification of Caustic
BRINE
PURIFICATION
• Ca, Fe and Mg compounds plug the
diaphragm
• Precipitation with NaOH is commonly
used to remove them
• Addditional treatment with phosphates
is required for membrane cells
• Sulphates may be removed by BaCl2.
• Brine is preheated with other streams
to reduce energy requirement.
BRINE
ELECTROLYSIS
• 3.0 – 4.5 V per cell is used; whichever
method is adopted
• Monopolar – Cells connected in parallel
and low voltage applied to each cell
• Bipolar – Cells are connected in series
and high voltage applied
EVAPORATION
AND SALT
SEPARATION
• 11 % NaOH (Diaphragm cells); 35% (Membrane
Cells) are concentrated to 50% NaOH in
multiple effect nickel tubed evaporators
• Salt crystallizes out and recycled
• Concentrated to 73% reduces shipping cost
but greatly increases the shipping and
unloading problems
• High m.p of conc material makes steamheated lines and steam heating of tank cars
necessary.
• Mp for 50% caustic 12°C; for 73%, 65°C.
EVAPORATION
AND SALT
SEPARATION
• Membrane cells produce more
concentrated caustic than diaphragm
cells
• Less Evaporation or treatment needed
(Membrane cell)
• Mercury cells produce 50% solution, so no
evaporation is needed
FINAL
EVAPORATION
• Cooled and settled 50% caustic may be
concentrated in a single-effect
evaporator to 70 – 75% NaOH using steam
at 500-600 kPa.
• Strong caustic must be handled in steamtraced pipes to prevent solidification
• It is run to finishing pots
• Another method – Treating 50% Caustic
solution with Ammonia
– Countercurrent system in pressure vessels
– Anhydrous crystals separate from resulting
aq. ammonia
FINISHING OF
CAUSTIC
• Dowtherm heated evaporators – removal of
water
• Product is pumped by a C.P that discharges
the molten material into thin steel drums or
into a flaking machine
SPECIAL
PURIFICATION
OF CAUSTIC
• Troublesome impurities in 50% caustic are
Fe, NaCl and NaClO3.
• Fe removed by treating caustic with 1%
CaCO3 and filtration
• NaCl and NaClO3 may be removed using
aq. NH3
• To further reduce salt content for some
uses; caustic is cooled to 20°C as shown in
following diagram
PURIFICATION
OF CAUSTIC
SODA
CHLORINE
AND
HYDROGEN
• Dried Chlorine is compressed to 240 or 550
kPa
• Lower pressure – rotary compressor
• Larger capacities and Pressures – Centrifugal and nonlubricated reciprocating compressors
• Heat of compression is removed and gas
condensed
• Liquid Cl is stored in small cylinders
• Hydrogen used in making other compounds
• With Cl  HCl
• Hydrogenation of fatty acids (Soap manufacture)
• Ammonia
SODA ASH
MANUFACTURE
Sodium Carbonate
SODA ASH
• Physical
– Odourless/hygroscopic; alkaline in nature
– Mp. 851 °C; M.wt = 106, Density @ 20 °C =
2.53 g/cm3;
• Chemical
– Thermal Decomposition at 1000 °C/200 Pa
–
Na2CO3  Na2O + CO2
– Lethal dose = 4g/kg (rat); 15g/kg human
USES OF SODA
ASH
•
•
•
•
•
Glass Industry
Water softening agent
Baking soda manufacture
Paper making
In Power generation to remove SO2 from
flue gas
MANUFACTURING
PROCESSES
Le Blanc Process
Solvay Process
LE BLANC
PROCESS
•
•
•
•
2 NaCl + H2SO4  Na2SO4 + 2 HCl
Na2SO4 + 2C  Na2S + 2 CO2
Na2S + CaCO3  Na2CO3 + CaS
Disadvantages
– Solid Phase
– Amount of energy
– CaS pollutant
LEBLANC
PROCESS
REACTION
SCHEME
LEBLANC
PROCESS
DIAGRAM
SOLVAY
PROCESS
• Continuous process using
limestone, ammonia and
NaCl to produce Na2CO3
SOLVAY
PROCESS
Brine
(NaCl)
Ammoniated
Brine
NaCl
H 2O
Limestone
CaCO3
Lime in
NH3
Carbonating
Tower
CO2
Kiln
H2O
CaO
Lime Slaker
Ammonia
Filter
Ca(OH)2
NaHCO3
300 °C
NH3
NH4Cl
Ammonia
Recovery
Waste by
product CaCl2
Product
1. Food additive
Na2CO3
2. Electrolyte
• Solvay Tower
REACTIONS
• 2 NH3 + CO2 + H2O  (NH4)2CO3 (exothermic)
• (NH4)2CO3 + CO2 + H2O  2 NH4HCO3
• NH4HCO3 + NaCl  NaHCO3 + NH4Cl2
Middle of Carbonator
• Lime Kiln
• CaCO3  CaO + CO2
• CaO + H2O  Ca(OH)2
• Calciner
• 2 NaHCO3  Na2CO3 + CO2 + H2O
• Ammonia Recovery
• 2 NH4Cl + Ca(OH)2  CaCl2 + 2 NH3 + 2 H2O
MANUFACTURING
STEPS
•
•
•
•
•
•
Brine Preparation
Ammonia Absorption
Precipitation of bicarbonate
Filtration of bicarbonate
Calcination of bicarbonate
Recovery of Ammonia
SOLVAY PROCESS
• NH3 Absorber
– Counter current flow; Baffles tray
– Cooler to remove heat of solution
– Slightly less than atm pressure
– Made of Cast iron
– At exit; NaCl = 260 g/l; NH3 = 80-90 kg/m3; CO2 = 40-50 kg/m3
• Carbonator
– 6 -9 in number; 20-30 m in height
– Exothermic reaction 60 °C
– To reduce solubility of NaHCO3 use cooler at bottom @ 30 °C
– Vacuum Rotary filter at bottom
THANK YOU!

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